For lazy people, use mongodump, it's faster:
mongodump -d <database_name> -o <directory_backup>
And to "restore/import" it (from directory_backup/dump/):
mongorestore -d <database_name> <directory_backup>
This way, you don't need to deal with all collections individually. Just specify the database.
Note that I would recommend against using mongodump/mongorestore for big data storages. It is very slow and once you get past 10/20GB of data it can take hours to restore.